The present invention relates to the extrusion of rubber products. More precisely, it relates to the manufacture of a composite profiled product formed of rubber mixes of different composition.
For the extrusion of raw rubber, the state of the art includes a type of apparatus known as a roller die extruder. It makes it possible to extrude a product of great length, the profile of which is constant. This profile, that is to say, the cross section of the extruded rubber product, is defined in this type of apparatus by, on the one hand, the roller onto which the rubber is extruded and, on the other hand, a fixed wall which cooperates with the roller in order to define an extrusion orifice.
Furthermore, the manufacture of tires requires at present the production of profiled semifinished products which are formed of rubber mixes having different properties, referred herein generally as "composite product". Thus, for example, it is necessary to produce a tread having a cross section which has a shape which approaches that which the tread will have when the tire is molded, and which is formed of mixes of different composition depending on the transverse location in the future tire. A roller die co-extrusion apparatus which produces a co-extrusion product by extruding each of the rubber mixes onto a different place on the surface of the roller so as progressively to form the profiled co-extrusion product is known from U.S. Pat. No. 4,539,169.
Another co-extrusion technique which makes it possible to manufacture a composite product is described in U.S. Pat. No. 2,382,177 in which a first mix is profiled by forcing it between two fixed walls, a second mix is profiled by creating a bead of rubber between a fixed wall and a movable wall, and the first mix is brought onto the second mix in the space between two rollers.
It is desired to manufacture tires in constantly more precise fashion. The precision of manufacture depends, in particular, on respect for the dimensions of the rubber products the assembling of which forms the tire. In other words, when a composite product is used upon the assembling of the tire, it is very important that the line or lines of separation between the rubber mixes of different composition be geometrically strictly respected. Now, it is known that the mechanical properties of rubber mixes are very poor before the vulcanizing of these mixes. In certain cases, the raw rubber mix is in the form of an extremely soft paste, while other rubber mixes may be relatively hard, even before vulcanization. When it is attempted to co-extrude a very soft rubber mix at the same time as a very hard rubber mix, it is difficult to control with perfection the geometry of the surface of separation between the two mixes of very different properties.
Despite the difficulties encountered, it is desirable to use composite products upon the assembling of the tire since this decreases the number of layings of products which it is necessary to effect during the stage of the assembling on the drum. Now, the number of different rubber mixes used to produce a single tire is tending to increase in order to be able to optimize the properties of these mixes as a function of the region of the tire in which they are located.